Add f-assistant NestJS RAG assistant service

[siarheihuzarevich]

Summary

• add f-assistant NestJS project skeleton for human-in-the-loop RAG assistant
• include ingestion pipeline, Telegram drafting flow, GitHub integration stubs, and Answer Pack validation
• provide docker-compose, Prisma schema, scripts, and example env for local setup

Testing

• not run (not requested)

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Codex Task